1. Field of the Invention
The invention relates generally to optical fibers and in particular to the drawing fabrication of birefringent fibers. More specifically, the invention relates to an optical technique for determination of the rotation of the principal axes during fabrication of the fiber, and the control of the fabrication apparatus to remove the cause of fiber rotation and thus reduce axes rotation.
2. Background Art
The performance of polarization maintaining couplers and devices in preserving high polarization-extinction ratios strongly depends upon the alignment of the principal birefringent axes ("PBA") of the polarization maintaining fiber ("PMF") with those of the device or a second PMF. Typical commercially-available PMF's have rates of internal rotation of the birefringent axes ranging from 0.44 to 11.degree./cm. The misalignment due to this internal rotation has been cited as limiting the polarization separation in tapered couplers to slightly better than -23 dB, while PMF's may support -45 dB separation.
The presence of undesirable rotation of the PBA in commercial specialty fibers was first recognized only as recently as 1985. Fabrication of fiber optic polarization-maintaining couplers and connectors depend on the alignment of the PBA, and polarized signal separation is degraded by a rapid rotation of the axes. Typical rotation rates range from 3 to 15 degrees per centimeter, and were first publicly recognized by the present inventors as being due to small misalignments in the mechanism used to draw the fiber. The performance of low birefringence fibers drawn with the same misalignments was not impaired.
Due to the recent recognition of the problem of PBA rotation, a technique has not yet appeared for its correction. The method by which the internal rotation of fiber axes was first established could be used to tune a draw tower (part of the drawing mechanism to draw the glass fiber preform) to address the defect, but it involves a time consuming technique which destroys the fiber sample being investigated. Such an approach would require a trial-and-error routine of drawing a length of fiber, destructively measuring the rotation rate and direction, and then adjusting the draw mechanism. The minute misalignment required to produce a significant rotation rate suggests that each new correction would only be stable and accurate for a short time before another test and adjustment would be necessary. It is therefore believed that no direct precedent for this invention exists.